To protect plant equipment and personnel, in addition to protecting neighboring property, people, and the environment, many industrial plants include emergency shutdown (ESD) valves designed to interrupt process flow in the event of an emergency, such as a pipe rupture. ESD valves are actuated valves that operate under fail-safe principles, forming part of a safety instrumented system. An ESD valve actuator drives its associated ESD valve through pneumatic or hydraulic pressure, with energy being stored with a compressed spring (in the case of single-acting actuators) or with energy being stored with compressed pneumatic or hydraulic fluid (in the case of double-acting actuators).
ESD valves require periodic testing to maintain their operability and to confirm that they are operable. However, a full stroke of the valve interrupts process flow, which is frequently impractical and economically undesirable. As a result, full stroke tests are typically only performed once every 3-5 years.
Another method of testing ESD valve operability that does not require the complete interruption of process flow is a partial stroke test. Partial stroke tests have historically driven ESD valves to approximately 10-30% closure. While not completely interrupting process flow, such a partial closure nevertheless has a significant impact on process variables. Partial stroke tests require that an operator be present at the facility, and preparatory steps need to be taken by operation, maintenance and inspection organizations prior to and during the test. As a consequence of the effect of a partial stroke test on process variables, and as a consequence of the manpower costs associated with partial stroke tests, they are typically only performed every 3-12 months.
There are a number of other disadvantages associated with the current partial-stroke devices. Some models can operate with a pneumatic actuator, and other models can operate with a hydraulic actuator, but there is no partial-stroke device that can interchangeably support both types of actuators. The main ESD solenoid valve cannot be subjected to a full trip test, but rather a separate solenoid valve that is independent from the safety loop is required for such a full trip test. Actuator quick exhausts cannot be tested with the current partial-stroke devices.
Another disadvantage is that the precise amount of closure (i.e., within the typical 10-30% closure range) is generally established for a particular valve based upon operating points and process variables that occur during a test. When the valve is repeatedly tested with a partial stroke to the same percentage of closure, this increases the possibility of creating a crest or ridge on the valve obturator. Depending on the process, this ridge could result from a number of sources, including pipe scaling, obturator coating, valve internal seals, hydrates, and precipitated elemental sulfur. Once a ridge on the valve obturator develops, all subsequent partial stroke tests may indicate the valve is functioning properly, while in reality, an actual closure demand may fail due to the valve obturator jamming on the seat, locking the ESD valve in place instead of going to its fail-safe position.
What is required is a system to automatically perform micro strokes of emergency shutdown valves, driving them to approximately 20-25% closure. The smaller level of closure will reduce the impact on process variables, and the automation of the test will reduce manpower expenses. The system should preferably allow for movements of 1% or less, depending on the resolution of the emergency shutdown valve and actuator, and will drive emergency shutdown valves to varying degrees of closure, avoiding the formation of ridges on obturators. The micro stroke system allows for a full trip in the event of an emergency shutdown demand. Such a system would therefore enhance the reliability of a critical safety system, while reducing costs over the long term.